Rails force models to eager load

Well, after some digging, it actually is really simple. Just need to run the following.

Rails.application.eager_load!

From Configuring Rails Applications

1. config.eager_load when true Eager loads all registeredconfig.eager_load_namespaces. This includes your application,engines, Rails frameworks and any other registered namespace.
2. config.eager_load_namespaces registers namespaces that are eagerloaded when config.eager_load is true. All namespaces in the listmust respond to the eager_load! method.
3. config.eager_load_paths accepts an array of paths from which Railswill eager load on boot if cache classes is enabled. Defaults toevery folder in the app directory of the application.

EDIT:

To manually load you should be able to do something like:

matcher = /\A#{Regexp.escape(load_path)}\/(.*)\.rb\Z/Dir.glob("#{load_path}/**/*.rb").sort.each do |file|  require_dependency file.sub(matcher, '\1')end

After a great deal of trial and error, I recently learned that Jason Waldrip's answer is somewhat incomplete. As of Rails 5, Rails.application.eager_load! will indeed load all of the directories specified in config/application.rb. But it doesn't match Rails' actual behavior in production. To do that, one must instead mirror what Rails does:

Rails.configuration.eager_load_namespaces.each(&:eager_load!)

The salient difference between the approaches is that OPs answer won't eager load the files within app directories of Engines that live in the gems or vendor folders. Whereas Rails itself will identify where subclasses of Engine exist and will see to it that the appropriate app subdirectories are eager-loaded.

Behind the scenes

Rails 5 adds eager load directories in railties-5.0.2/lib/rails/engine/configuration.rb:39, where it runs

      paths.add "app",                 eager_load: true, glob: "{*,*/concerns}"      paths.add "app/assets",          glob: "*"      paths.add "app/controllers",     eager_load: true      paths.add "app/channels",        eager_load: true, glob: "**/*_channel.rb"      paths.add "app/helpers",         eager_load: true      paths.add "app/models",          eager_load: true      paths.add "app/mailers",         eager_load: true

These directories are not currently included in a default Rails.application.eager_load!